1. Field of the Invention
This invention relates to information retrieval apparatus and, more particularly, to an improved transducer head adapted to recover the video information which has been stored on the surface of a video disc.
2. Description of the Prior Art
It has been known that color video programs can be stored on magnetic tape for subsequent playback, utilizing appropriate video tape reading apparatus. It has also been disclosed that the same information can be recorded on a disc, either through photographic or other processes which achieve a physical defomation of the disc in a predetermined pattern, which can then optically "read" to recover a video signal suitable for application to a standard TV receiver.
In the teachings of the prior art, and especially the patents to Gregg and Johnson and the application of Elliott, supra, apparatus has been disclosed which is adapted to cooperate with the video disc disclosed by Gregg, Johnson, Broadbent, et al, supra. Continued experimentation has led to the development of video disc having a surface upon which information is stored as "holes" or depressions in a surface.
On a typical video disc, a given hole may be approximately 1 micron in width. A plurality of such holes of varying lengths are placed in a more or less continuous track on a surface of a disc. The disc is adapted to rotate at approximately 1800 rpm for playback. In alternative configurations, the disc may either have a continuous spiral track containing information or may include a plurality of discrete, circular tracks. In either embodiment, a track-to-track spacing of approximately 2 microns is maintained. At such rotational speeds, approximately 20 minutes of program can be accommodated on a 12 inch diameter disc.
In order to provide a commercially successful system, several requirements must be simultaneously satisfied. The video disc which contains the program material must be easily mass produced, able to take a certain amount of handling and must function on a playback instrument which must be reasonable in cost and sufficiently simple and rugged so that it might function in the environment of a home.
Techniques are available to mass produce discs using techniques which are analogous to those employed in the phonograph record industry. It has been determined that a video disc can be made of thin material without substantial lateral rigidity. Such a disc can accept information in a "hole-no-hole" pattern which can be optically recognized by suitable playback equipment. However, the flexibility of the thin plastic disc imposes certain requirements on the playback equipment. Because of the microscopic size of the tracks and the information recorded therein, an optical system must be provided which can discriminate between the presence or absence of a "hole" 1 micron wide in a series of similar holes.
The series of holes of interest is separated from an adjacent series of holes or track by approximately 1 micron, since the distance between the centers of adjacent tracks is in the preferred embodiment approximately 2 microns.
Sufficient energy must be applied to and recovered from the surface to distinguish between the surface states that represent information, and to provide an error signal which enables a control system to maintain the transducer in alignment with the track of interest in an environment of shocks and vibration.
In order to resolve, optically, a spot that is 1 micron in width, the distance between the object plane and the optical system should be held constant to within approximately a micron. If now the surface of the disc cannot be held planar within a micron, it is necessary to provide some mechanism that will preserve the spacing between the disc surface and a predetermined point in the optical system.
In the prior art, utilizing a substantially rigid disc, a reading head assembly had been disclosed which included a hydrodynamic or fluid bearing which, in conjunction with a mechanical force biasing the head towards the disc surface, maintained the head at a fixed distance from the disc with acceptable accuracy. With a nonrigid, flexible disc, it is necessary first to define the object plane in which the information track is found and then to provide a mechanism that maintains the spacing between that object plane and the optical system.